The present invention is related to a torque screwdriver equipped with a scale for checking torque value.
A conventional torque screwdriver has a main body in which a female clutch shaft is fitted. An end face of the female clutch shaft is inward recessed to form a tapered recess. A male clutch shaft is formed with a tapered head section adapted to the recess. A spring is mounted between the head section and the socket.
In operation, the main body is turned and an axial force is applied to the male clutch shaft. At this time, the periphery of the head section of the male clutch shaft tightly abuts against the wall of the recess of the female clutch shaft. Accordingly, by means of rotating the male clutch shaft, the female clutch shaft is driven and rotated under frictional force. At this time, a torque is output to drive a work piece.
However, the force exerted onto the female clutch shaft by the male clutch shaft depends on the force applied by an operator. The application force can be hardly unified so that the frictional force between the male and female clutch shafts is always uncertain. As a result, the torque output by the female clutch shaft is varied and the screw can be hardly tightened to a fixed extent. Generally, in the case that the screw is too loose, the screw is apt to detach and drop. In some serious cases, the screw will loosen to lead to damage of the entire machine or even loss of lives.